This project concerns the basic adaptive mechanisms of the eye that maintain visual sensitivity. The direct objective is to understand how the Limulus lateral eye adapts to changes in the illumination of the natural environment and why an underlying daily cycle of structural and functional changes is essential to its function. We plan a systematic study involving both the experimental techniques of neurophysiology and the histological procedures of light microscopy, electron microscopy, and autoradiography. We first will investigate the changes in the structure and ultrastructure of the retina as it integrates the effects of light and of circadian efferent inputs from the brain. Efferent effects will be separated from those due to light by manipulating experimental parameters. Functional changes will be correlated with structural changes directly by performing both physiological and anatomical experiments on the same single photoreceptor. Emphasis is placed on the process of photosensitive membrane turnover and the mechanisms which control it. Our long range goal is to develop from the Limulus model system ideas which have general application particularly with regard to more complex visual systems. We wish to elucidate the nature, purpose, and control of membrane turnover in photoreceptors. These results may provide some insight into retinal pathologies such as retinitis pigmentosa which appear to involve a breakdown of the control mechanisms for membrane turnover in human rods and cones. Limulus has already provided a promising start in our attempts to understand the nature of photosensitive membrane turnover and its control mechanisms.